Near field magnetic coupling (NFMC) is a commonly employed technique to wirelessly transfer electrical energy. The electrical energy may be used to directly power a device, charge a battery or both.
In near field magnetic coupling (NFMC) an oscillating magnetic field generated by a transmitting antenna passes through a receiving antenna that is spaced from the transmitting antenna, thereby creating an alternating electrical current that is received by the receiving antenna.
However, when a magnetic field interacts with an electrically conductive material, such as a metal, a circulating eddy current is induced on the surface of the conductive material as described by Faraday's Law. The eddy current generated on the surface of the conductive material generates a secondary magnetic field which opposes an incident magnetic field generated by the transmitting antenna. The interaction between the incident magnetic field generated by the transmitting antenna and the opposing secondary magnetic field, generated by the eddy current induced in the metallic material, prevents the transmission of electrical energy from the transmitting antenna to the receiving antenna. Thus, the inclusion of a metallic substrate or backing with electronic devices that incorporate near field magnetic coupling wireless charging is prohibited. The interaction of the incident magnetic field with a metallic material comprising an exterior surface of a device prevents near-field wireless electrical energy transmission with the device.
In addition, the circulating eddy current causes undesirable heating of the conductive material. Such heating may result in the loss of wireless energy but also could result in damage to the electrical components within the device due to the increased heat. Currently, materials in which NFMC wireless energy can transmit through are limited to non-conductive materials, such as glass, that do not deleteriously affect the incident magnetic flux lines from a transmitting antenna. Incorporation of an electrically conductive material, such as a metal, within at least a portion of an exterior surface of a device, may be required. Electrically conductive materials provide structural support to such electronic devices as cellular phones, wearable devices, and medical devices, all of which could benefit from wireless transfer of electrical energy through NFMC. Thus, there is a need for NFMC wireless energy transfer through a metallic material.